Optical system and method for transmitting a source image

ABSTRACT

An optical system for transmitting a source image is provided. Light having a field angle spectrum emanates from the source image. The optical system includes an optical waveguide arrangement, in which light can propagate by total internal reflection. The optical system also includes a diffractive optical input coupling arrangement for coupling the light emanating from the source image into the optical waveguide arrangement. The optical system further includes a diffractive optical output coupling arrangement for coupling the light that has propagated in the optical waveguide arrangement out from the optical waveguide arrangement. The disclosure also provides related methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims benefit under35 USC 120 to, international application PCT/EP2016/080902, filed Dec.14, 2016, which claims benefit under 35 USC 119 of German ApplicationNo. 10 2015 122 055.5, filed Dec. 17, 2015. The entire disclosure ofthese applications are incorporated by reference herein.

FIELD

The disclosure relates to an optical system and method for transmittinga source image.

BACKGROUND

An optical system and a method for transmitting a source image aredisclosed in the article by Tapani Levola: “Diffractive optics forvirtual reality displays”, Journal of the SID 14/5, 2006, pages 467 to475. Further optical systems and methods for transmitting a source imageare known from US 2014/0140653 A1, U.S. Pat. No. 8,233,204 B1, US2014/0218801 A1, which disclose an optical system according to thepreamble of patent claim 1. A further optical system for transmitting asource image is known from US 2006/0221448 A1.

An optical system of this type and a method of this type are used forexample in a display system, such as an HUD (head-up display) or HMD(head-mounted display). An HUD and an HMD are display systems in which asource image is projected into the user's field of view withmagnification of the exit pupil. In this case, the source image can befor example the image of a display of a vehicle instrument, of acellular phone, of a games console, of a computer and the like. HUDs areused nowadays for example in aircraft and motor vehicles in order toproject information, for example navigation information and the like,for the pilot or driver into the latter's field of view, without thepilot or driver having to divert his/her viewing direction from thestraight-ahead viewing direction. An HMD, in contrast to an HUD, is wornon the user's head. An HMD either presents images on a screen close tothe eyes, or projects the images directly onto the retina. Otherdesignations for an HMD include video glasses or smartglasses, helmetdisplay or virtual reality helmet.

The principal components of such display systems are a display unit,which supplies the source image from a connected data source, and anoptical system for transmitting the source image into a target image.

One important characteristic variable of such display systems is thefield of view (FOV). The field of view of such display systems desirablyhas a magnitude such that the entire source image is transmitted intothe target image. The field of view is the difference between themaximum and minimum angles, in each case measured from the center of theimage to the mutually opposite image edges in a horizontal dimension(horizontal field of view) and in a vertical dimension (vertical fieldof view). In the present description, reference is made only to thefield of view in one dimension.

The optical system of such display systems, as is disclosed in thearticle cited above, have as component parts an optical waveguidearrangement having one or more optical waveguides, in which opticalwaveguide arrangement light can propagate by total internal reflectionat optical interfaces, a diffractive optical input coupling arrangement,by which the light emanating from the source image can be coupled intothe optical waveguide arrangement, and a diffractive optical outputcoupling arrangement, by which the light that has propagated in theoptical waveguide arrangement can be coupled out from the opticalwaveguide arrangement, such that the light can enter one or both of theuser's eyes. In this case, the optical waveguide arrangement can haveone or more optical waveguides, and the input coupling arrangement andthe output coupling arrangement can have one or more diffractiongratings.

In general, in the case of optical systems having the constructiondescribed above, it has been found that the field of view of such anoptical system is restricted, that is to say that the entire sourceimage or, in other words, the entire field angle spectrum of the lightemanating from the source image cannot be transmitted by the opticalsystem. In the case of relatively large source images, for example inthe 16:9 format that is customary nowadays, edge regions may be absentin the transmitted image.

Generally, the field of view is small in the case of optical systemshaving the construction described above. In the case of HMDs, inparticular, there is by contrast the desire for the largest possiblefield of view with image angles of the field of view of more than 20°,and preferably more than 40°.

During the transmission of polychromatic source images, as is the casefor example during the transmission of video images, encompassing theentire visible spectrum in the wavelength range of approximately 425 nmto approximately 675 mm, the further issue can arise that the field ofview becomes all the smaller, the larger the spectral range to betransmitted. In general, the field of view is restricted, however, evenduring monochromatic transmission.

During polychromatic transmission, on account of the wavelengthdependence of the diffraction, this can additionally have the effectthat the transmitted source image does not have color fidelity relativeto the source image to be transmitted because for example the entirewavelength spectrum of the source image is not transmitted into thetarget image or different spectral ranges having different intensitiesare transmitted.

A further property of such an optical system that can restrict the fieldof view involves propagation of the light in the optical waveguidearrangement by total internal reflection. This type of light propagationis present, however, if the optical system is intended to be transparentat least in the user's field of view, as is desired in the case of anHUD or HMD, in particular smartglasses, such that the user can see thetransmitted source image in superimposition with the real world.Generally, total internal reflections within the optical waveguidearrangement occur, however, only if the light incident on the opticalinterface(s) of the optical waveguide arrangement has an angle ofincidence that is greater than the critical angle of total internalreflection.

SUMMARY

The disclosure seeks to develop or specify an optical system and amethod to the effect that they make it possible to realize displaysystems having a larger field of view than the display systems availablein the prior art.

According to the disclosure, the first subfield and/or the at least onesecond subfield are/is at least partly arcuately bounded before couplinginto the optical waveguide arrangement.

Furthermore, the disclosure specifies an optical method for transmittinga source image, wherein light having a field angle spectrum emanatesfrom the source image, including the following steps:

-   -   dividing the light emanating from the source image into a first        subfield, wherein the light of the first subfield has field        angles in a first field angle range of the field angle spectrum,        and into at least one second subfield, wherein the light of the        at least one second subfield has field angles in a second field        angle range—different from the first field angle range—of the        field angle spectrum;    -   diffractively coupling in light of the first subfield and,        separately from the input coupling of the light of the first        subfield, diffractively coupling in the at least one second        subfield into an optical waveguide arrangement and propagating        the light of the first subfield and of the second subfield in        the optical waveguide arrangement, wherein the first subfield        and/or the at least one second subfield are/is at least partly        arcuately bounded; and    -   diffractively coupling out the first subfield and the at least        one second subfield from the optical waveguide arrangement in        such a way that the transmitted first subfield and the        transmitted at least one second subfield of the source image are        superimposed on one another after coupling out from the optical        waveguide arrangement.

In the optical system according to the disclosure and the methodaccording to the disclosure, the entire field of the source image havingthe entire field angle spectrum is not coupled into the opticalwaveguide arrangement via one individual input coupling element, ratherthe source image is divided into at least two subfields, wherein arespective dedicated input coupling element is assigned to theindividual subfields, and wherein the individual subfields are coupledinto the optical waveguide arrangement separately from one another. Onlythe first subfield is coupled in via the first input coupling element,and only the at least one second subfield is coupled in via the at leastone second input coupling element. The subfields can be coupled into acommon optical waveguide, or a dedicated optical waveguide can beassigned to each subfield. The input coupling elements for the differentsubfields can overlap one another or be arranged at the opticalwaveguide arrangement without overlapping one another. After theindividual subfields have propagated through the optical waveguidearrangement, the subfields are coupled out from the optical waveguidearrangement via the output coupling arrangement and superimposed on oneanother, such that the user sees the complete transmitted source image.

According to the disclosure, the first subfield and/or the at least onesecond subfield are/is at least partly arcuately bounded before couplinginto the optical waveguide arrangement. The source image, for example adisplay, is usually rectangular in shape. On account of the opticalconditions during the transmission of the source image via totalinternal reflection in the optical waveguide arrangement, only a smallerfield angle range can be transmitted if the source image were split intorectangular subfields. By virtue of the source image being splitaccording to the disclosure into subfields, at least one of which isarcuately bounded, it is possible by contrast, as will be describedlater, to transmit a larger field angle range and thus to achieve alarger field of view.

With the optical system according to the disclosure and the methodaccording to the disclosure, it is possible in this way to achievesignificantly larger fields of view (FOVs) than with the known opticalsystems. The disclosure enables a larger field of view and/or thetransmission of a larger wavelength range in an individual transmissionchannel. In the prior art, by contrast, the transmission of a largerwavelength range is accompanied by the restriction of the field of viewto a smaller field of view. The optical system according to thedisclosure and the method according to the disclosure, by contrast,enable the transmission of large wavelength ranges without a loss inrespect of the size of the field of view.

The first subfield has a first field edge and the at least one secondsubfield has a second field edge, wherein the first field edge isdirectly adjacent to the second field edge, and wherein the first fieldedge is concavely arcuate and the second field edge is convexly arcuate,or wherein the first field edge is convexly arcuate and the second fieldedge is concavely arcuate.

In this case, the first subfield and the second subfield can have apartial overlap in the region of the first and second field edges ifthis is desired in order, after the individual subfields have beencoupled out from the optical waveguide arrangement, to obtain a completetransmitted source image by stringing together the subfields. This isbecause the radii of the arcuate field edges may be slightly differentfrom subfield to subfield, and so a slight overlap avoids a loss ofimage information.

Furthermore, the input coupling arrangement can have a third inputcoupling element for coupling in a third subfield of the source image,wherein the third subfield is arranged between the first and secondsubfields, and wherein the third subfield has third field edges, ofwhich one is directly adjacent to the first field edge and the other isdirectly adjacent to the second field edge, and wherein both third fieldedges are arcuate.

In this configuration, the source image is thus split into at leastthree subfields, of which the outer subfields are arcuately bounded atleast at one side and the central subfield is arcuately bounded on bothsides. The individual arcuate field edges are arcuate approximatelycomplementarily to one another in each case in such a way that if onefield edge is concavely arcuate, the field edge directly adjacentthereto is convexly arcuate, or vice versa.

In the context of the present disclosure, however, it is not onlypossible to split the source image into subfields whose mutuallyadjacent field edges are arcuate, but it is likewise possible for thefirst subfield and/or the second subfield to be arcuately bounded at anouter field edge. In this configuration, accordingly, mutually adjacentfield edges of the at least two subfields are not or not only the onesthat are arcuate, rather at least one field edge of at least one of thesubfields which is not adjacent to a field edge of the other subfield isarcuate.

Although a larger field of view by comparison with the prior art canalready be achieved if the source image overall is divided into only twosubfields, each of which is individually transmitted, generallyprovision is made for the input coupling arrangement to have a number ofN input coupling elements, wherein N is an integer ≥2, which arearranged for coupling light from N different subfields of the sourceimage having field angles from N different field angle ranges of thefield angle spectrum into the optical waveguide arrangement.

Splitting the source image into more than two subfields has theadvantage that particularly large fields of view having a large spectralbandwidth can be achieved. However, more input coupling elements arethen also used, and the optical waveguide arrangement then possibly hasa larger number of optical waveguides, which may lead to a highercomplexity of the optical system.

A further improvement in the optical system according to the disclosureis achieved by virtue of the fact that the input coupling arrangementhas at least two first and at least two second input coupling elements,wherein one of the two first and one of the two second input couplingelements are arranged for coupling light from the first subfield andfrom the second subfield, respectively, in a first wavelength range intothe optical waveguide arrangement, and the other of the two first andthe other of the two second input coupling elements are arranged forcoupling light from the first subfield and from the second subfield,respectively, in a second wavelength range, which is different from thefirst wavelength range, into the optical waveguide arrangement.

In the case of this measure, in addition to the spatial division of thesource image into a plurality of subfields, a division of the totalspectral range emanating from the source image into different spectralsubranges (“different color channels”) takes place as well. With thismeasure, the disadvantageous effects described in the introductionregarding the wavelength dependence of diffraction can be reduced oreven eliminated given appropriate design of the input coupling elements,such that overall a polychromatic transmission of the source image witha large field of view can be achieved.

Preferably, in the case of the measure mentioned above, the wavelengthspectrum is subdivided into three wavelength ranges, preferably intored, green and blue. The division of the total spectral range of thelight emanating from the source image can be achieved via spectralfilters, for example.

In a preferred development of the measure mentioned above, the opticalwaveguide arrangement has a first optical waveguide for the propagationof light in the first wavelength range and has at least one separatesecond optical waveguide for the propagation of light in the at leastone second wavelength range.

In accordance with this configuration, therefore, separate transmissionchannels are used for the transmission of the different wavelengthranges. In this case, the individual optical waveguides are preferablyarranged or stacked one above another transversely with respect to thelight propagation direction in the optical waveguides.

In a further preferred configuration, the optical waveguide arrangementhas a first optical waveguide, into which the first input couplingelement couples the light from the first subfield, and at least onesecond optical waveguide, into which the at least one second inputcoupling element couples the light from the at least one secondsubfield.

In this configuration, also for the transmission of the individualsubfields of the source image in each case a dedicated transmissionchannel is used, which can be arranged in a manner lying one aboveanother.

In connection with the abovementioned measure of the spectral divisionof the source image light into three color channels and in the casewhere the source image is spatially divided into two subfields, thisresults overall in 2×3, i.e. six, transmission channels for the sourceimage, corresponding to the division of the source image into twosubfields and the division of the wavelength spectrum into threewavelength ranges.

As an alternative to the configuration mentioned above, however, theoptical waveguide arrangement can also have an optical waveguide, intowhich the first input coupling element couples the light from the firstsubfield and the at least one second input coupling element couples inthe light from the at least one second subfield, wherein the first inputcoupling element and the second input coupling element are arranged inopposite end regions of the optical waveguide.

In a configuration similar thereto, the optical waveguide arrangementcan have an optical waveguide, into which the first input couplingelement couples the light from the first subfield and the at least onesecond input coupling element couples the light from the at least onesecond subfield, wherein the optical waveguide has two mutually parallelfirst and second sections and at a first end a third sectionperpendicular to the first and second sections, and wherein the firstand second input coupling elements are arranged at free second ends ofthe optical waveguide.

In these configurations it is advantageous that the optical waveguidearrangement overall has fewer optical waveguides or layers, which leadsto a thinner design transversely with respect to the light propagationin the optical waveguide arrangement. A further advantage of theseconfigurations is that reflection losses during the transmission of thesource image are reduced. Yet another advantage is that the outputcoupling arrangement can manage with fewer output coupling elements.

In a further preferred configuration, the output coupling arrangementhas a first output coupling element, which is arranged for couplinglight from the first subfield of the source image out from the opticalwaveguide arrangement, and at least one second output coupling element,which is arranged for coupling light from the at least one secondsubfield of the source image out from the optical waveguide arrangement.

In this configuration, the at least two subfields of the source imageare coupled out from the optical waveguide arrangement via outputcoupling elements specifically assigned to the respective subfield. Thishas the advantage, in particular, that the respective output couplingelement can be individually adapted in relation to the associated inputcoupling element, in particular can be configured symmetrically withrespect thereto or at least with the same grating period.

In connection with one of the configurations mentioned above, accordingto which the optical waveguide arrangement has at least two opticalwaveguides for the transmission of the at least two subfields, it isfurthermore preferred if the first output coupling element couples lightout from the first optical waveguide and the second output couplingelement couples light out from the second optical waveguide.

In connection with one of the measures mentioned above, according towhich the at least two subfields are transmitted via a common opticalwaveguide, it is preferably provided that the first output couplingelement and the second output coupling element couple light out from theone optical waveguide.

In this connection it is furthermore preferred if the output couplingarrangement has a plurality of first output coupling elements and aplurality of second output coupling elements, wherein the first andsecond output coupling elements are arranged alternately along theoptical waveguide.

The measure mentioned above has the advantage of a particularly compactdesign of the optical system.

In a further preferred configuration, which is advantageous inparticular in combination with one or more of the configurationsmentioned above, according to which the optical waveguide arrangementhas a common optical waveguide, into which the at least two subfieldsare coupled, provision is made for the output coupling arrangement tohave only one first output coupling element.

In this case, it is advantageous that the number of output couplingelements for the optical system to be reduced.

In further preferred configurations, the first input coupling elementand/or the at least one second input coupling element have/has atransmissive optical diffraction grating structure or a reflectiveoptical diffraction grating structure.

In the same way, the first output coupling element and/or the at leastone second output coupling element can have a transmissive opticaldiffraction grating structure or a reflective optical diffractiongrating structure.

Reflective optical diffraction grating structures, in particular in theform of metallic gratings, during input coupling have the advantage of ahigher input coupling efficiency, which is also homogenous over theangle of incidence. For the TM (transverse magnetic) polarization, theinput coupling efficiency is virtually constant, such that a polarizedinput coupling could be preferred. On the other hand, the use ofpolarized light means a loss of 50% of the intensity, and unpolarizedinput coupling is significantly simpler. Since the diffraction gratingstructure effects polarization, however, after the input coupling amixture of the two polarizations can be achieved by virtue of beamsplitters being fitted at or between the optical waveguides, such thatthe beam paths pass repeatedly through the beam splitters, whichtransmit in each case 50% into each polarization and thus bring about amixture of both polarizations.

In a further preferred configuration, the diffraction grating structurehas webs that are inclined relative to the grating base, or thediffraction grating structure is a blazed grating.

Diffraction grating structures having inclined webs and blazed gratings,for the optical system of a display system, are preferable to binarygratings with regard to their input coupling and/or output couplingefficiency. Blazed gratings, in particular, have the advantage thattheir diffraction efficiency can be maximal in a specific order ofdiffraction and minimal in the other orders of diffraction.

In the case where the output coupling arrangement has only one outputcoupling element, as may be provided in a configuration mentioned above,the output coupling element can have the same grating period, but avariable shape of the grating structures, over its extent along theoptical waveguide, in order that the two subfields that are transmittedvia the one optical waveguide are thus coupled out in a suitable manner.

In this case, provision can be made for the shape of the gratingstructures to be symmetrical in a central region of the output couplingelement and to be increasingly asymmetrical in regions on both sides ofthe central region.

By way of example, the central region of the output coupling element canhave a sinusoidal grating structure having the same output couplingefficiency for both subfields, while in the regions on both sides of thecentral region the grating structure is increasingly blazed and theoutput coupling efficiency is optimized for respectively one of thesubfields, wherein the inclination of the blazed grating structures inthe two regions on both sides of the central region is opposite to oneanother.

In a further preferred configuration, the diffraction grating structureof the first input coupling element and the diffraction gratingstructure of the first output coupling element have the same gratingperiod, and/or the diffraction grating structure of the second inputcoupling element and the diffraction grating structure of the secondoutput coupling element have the same grating period.

As a result of this measure, a symmetry between input and outputcoupling is achieved, that is to say that a light ray that is incidenton the input coupling arrangement from the source image at a specificfield angle is coupled out from the optical waveguide arrangement viathe output coupling arrangement at the same field angle. In other words,identical angles of incidence are transmitted into identical angles ofemergence.

In a further preferred configuration, the optical system has a devicefor dividing the source image field into the first and at least onesecond subfield.

Such a device can be realized by beam deflecting elements in proximityto the source image in order to divide the light emanating from thesource image into a plurality of subfields. The advantage here is thatthe source image prior to coupling into the optical waveguidearrangement need not be duplicated in a number corresponding to thenumber of subfields. This last is likewise possible, however.

In particular, it is preferred if the device for dividing the sourceimage has an optical arrangement having at least one field stop forgenerating the first and/or second subfield having an at least partlyarcuate field edge.

For this purpose, the source image can be provided in a correspondingnumber of duplications, wherein for generating a respective one of thesubfields use is made of a corresponding field stop that masks out apart of the source image with a bent field edge.

Provision may likewise be made, however, for the device for dividing thesource image to have an electronic device for electronically generatingthe first and/or second subfield having an at least partly arcuate fieldedge.

In this case, the pixels of the source image (display) are driven suchthat a portion of the pixels remains dark, wherein the arrangement ofdark pixels is arcuately bounded.

In a further preferred configuration, the optical waveguide arrangementis planar.

The advantage here is that the optical waveguide arrangement and inparticular the coordination of the input coupling elements and theoutput coupling elements can be realized in a simple manner because nowavefront aberrations caused by curvature of the optical waveguidearrangement need be taken into account.

In an alternative preferred configuration, the optical waveguidearrangement is curved.

The advantage here is that the optical system including a curved opticalwaveguide arrangement can be integrated more simply into glasses worn bythe user, for example if the display system in which the optical systemis used is configured as an HMD.

In connection therewith there is the problem, however, that, owing tothe curvature of the optical waveguide arrangement, wavefrontaberrations occur which are caused by the total internal reflections ofthe light at the curved interfaces of the optical waveguide arrangement.

In a preferred development of the measure mentioned above, the opticalwaveguide arrangement has between the input coupling arrangement and theoutput coupling arrangement a diffractive correction arrangement inorder to correct aberrations of the wavefront of the transmitted light.

Wavefront aberrations, be they geometric and/or chromatic in nature,which are caused by the curvature of the optical waveguide arrangement,are compensated for via the diffractive correction arrangement, whichhas a correction diffraction grating, for example. The light propagatingalong the curved optical waveguide arrangement between the inputcoupling element and the output coupling element “sees” a planar opticalwaveguide arrangement on account of the diffractive correctionarrangement. This in turn has the advantage that the relations of aplanar optical waveguide arrangement can be taken as a basis for thecalculation of the input coupling and output coupling element(s).

The abovementioned aspect of a curved optical waveguide with acorrection arrangement is also regarded as an independent disclosure,specifically not only in conjunction with a diffractive input couplingarrangement and a diffractive output coupling arrangement, but also witha non-diffractive input coupling arrangement or output couplingarrangement, and likewise without the spatial division of the sourceimage into a plurality of subfields, and without the arcuate boundary ofthe subfield(s).

In further configurations of this aspect, the correction arrangement canbe arranged only in sections or locally along the curved opticalwaveguide arrangement. In this case, the correction arrangementcompensates for the cumulative wavefront aberrations that can arise atthe plurality of reflections in the curved optical waveguidearrangement. Alternatively, however, the correction arrangement can alsobe embodied over the entire area of the curved optical waveguidearrangement. This enables a piecewise compensation of the wavefrontaberrations generated in the curved optical waveguide arrangement, forexample of the wavefront aberration generated by the respectivelydirectly preceding or directly succeeding reflection in the curvedoptical waveguide arrangement. In particular, aberrations that arise onaccount of the curved optical waveguide from one output couplinglocation to the next are thus corrected by the correction arrangement.Thus, within a region in which light is coupled out via a plurality ofoutput coupling locations, a complete correction of the imaging beampath is ensured and a corrected image of the source image is offered tothe observer at each output coupling location.

The correction arrangement can be designed either for the inner (inrelation to the radius of curvature) or for the outer surface or forboth surfaces of the curved optical waveguide arrangement or of thecurved optical waveguide. The effect of the curved optical waveguidearrangement on wavefront aberrations and possibly induced chromaticaberrations can be corrected in the same grating or in a second gratingof the correction arrangement.

Further advantages and features are evident from the followingdescription and the accompanying drawing.

It goes without saying that the aforementioned features and those yet tobe explained below may be used not only in the respectively specifiedcombination but also in other combinations or on their own, withoutdeparting from the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the disclosure are illustrated in the drawingand described in more detail below with reference thereto. In thefigures:

FIG. 1 shows a basic schematic diagram for elucidating the physicalrelationships of the diffractive input coupling of two light rays intoan optical waveguide and the diffractive output coupling thereof for twodifferent wavelengths;

FIG. 2 shows an optical waveguide with one input coupling arrangementand two output coupling arrangements in accordance with the prior art;

FIG. 3 shows a diagram showing the maximum ratio of the step lengthbetween total internal reflections of a light ray having a longwavelength to the step length between total internal reflections of alight ray having a short wavelength within an optical waveguide fordifferent materials of optical waveguides as a function of the field ofview;

FIG. 4 shows a further basic schematic diagram for elucidating physicalrelationships in the case of diffractive input coupling of light into anoptical waveguide and propagation of the light via total internalreflection in the optical waveguide;

FIG. 5 shows a diagram showing the maximum transmittable wavelengthrange as a function of the field of view;

FIG. 6 shows a diagram showing the minimum refractive index of thematerial of an optical waveguide as a function of the field of view;

FIG. 7 shows a diagram, as in FIG. 5, showing the maximum transmittablewavelength range as a function of the field of view;

FIG. 8 shows a basic schematic diagram of an optical system with colormultiplexing;

FIG. 9 shows one exemplary embodiment of an optical system according tothe disclosure;

FIG. 10 shows a further exemplary embodiment of an optical systemaccording to the disclosure;

FIG. 11 shows yet another exemplary embodiment of an optical systemaccording to the disclosure;

FIGS. 12A-E show still further exemplary embodiments of an opticalsystem according to the disclosure in accordance with a further aspect;

FIG. 13 shows a basic schematic diagram of two formats of transmitted ortransmittable source images;

FIG. 14 shows a basic schematic diagram of an optical system fortransmitting a source image;

FIG. 15 shows two basic schematic diagrams of a field angle spectrum ink-space for illustrating transmittable field angle ranges of a sourceimage;

FIG. 16 shows a further basic schematic diagram for elucidating therelationships of transmitted field angles in k-space;

FIG. 17 shows further basic schematic diagrams for illustratingtransmittable field angles in k-space;

FIG. 18 shows a basic schematic diagram similar to the lower schematicdiagram in FIG. 17 with a depicted source image;

FIG. 19 shows six basic schematic diagrams corresponding to the basicschematic diagrams in FIG. 17 for three optical waveguides arranged onebehind another in the viewing direction of an observer and having threedifferent grating periods of a respective output coupling grating;

FIG. 20 shows three basic schematic diagrams of different, slightlyoverlapping subfields in accordance with the basic schematic diagrams inFIG. 19;

FIG. 21 shows two further basic schematic diagrams for illustratingarcuately bounded field angle ranges during the transmission of a sourceimage;

FIG. 22 shows a basic schematic diagram of an optical system fortransmitting a source image including three optical waveguides, threeinput coupling elements and three output coupling elements;

FIGS. 23A-B show devices for generating a subfield from a source imagehaving an arcuate boundary in two embodiment variants;

FIG. 24 shows a basic schematic diagram similar to the right-hand basicschematic diagram in FIG. 20 for illustrating the effect if the sourceimage is not divided into a plurality of arcuately bounded subfields;

FIG. 25 shows a basic schematic diagram of an optical system fortransmitting a source image in accordance with a further aspect;

FIG. 26 shows a basic schematic diagram of a further optical system fortransmitting a source image with end-side input coupling of thesubfields of the source image;

FIG. 27 shows a central segment of the optical system from FIG. 26 forillustrating a configuration possibility of an output coupling elementof the optical system in FIG. 26;

FIG. 28 shows a basic schematic diagram for illustrating thetransmittable field angle ranges of the optical system in FIG. 26 in arepresentation in k-space;

FIG. 29 shows three basic schematic diagrams of the stringing togetherof transmitted subfields in k-space in accordance with the transmittablefield angle ranges in FIG. 28; and

FIG. 30 shows a basic schematic diagram of a further optical system fortransmitting a source image in accordance with a further aspect.

DETAILED DESCRIPTION

In order to afford a better understanding of the configurationsaccording to the disclosure of optical systems for transmitting a sourceimage, which configurations will be described later, firstly thephysical relationships of diffractive input coupling of light into anoptical waveguide, the propagation of the light in the optical waveguideand the diffractive output coupling of the light from the opticalwaveguide will be explained with reference to FIGS. 1 to 8.

FIG. 1 firstly shows an optical waveguide arrangement 100 including anoptical waveguide 102, which is configured as a plane-parallel plate. Adiffractive input coupling arrangement 104 and a diffractive outputcoupling arrangement 106 are arranged at the optical waveguidearrangement 100.

The diffractive input coupling arrangement 104 has a transmissivediffraction grating structure 108, which is configured for example as ablazed grating and which is arranged at a first surface 110 of theoptical waveguide 102.

The output coupling arrangement 106 has a diffraction grating structure112, which is arranged at a surface 114 situated opposite the surface110 and is likewise transmissive.

Furthermore, FIG. 1 shows two light rays 116, 118 that are incident onthe input coupling arrangement 104 perpendicularly. Both light rays 116and 118 are coupled into the optical waveguide 102 with diffraction atthe input coupling arrangement 104. Within the optical waveguide 102,the two light rays 116, 118 propagate by total internal reflection atthe surfaces 110 and 114. As soon as the light rays 116, 118 reach theoutput coupling arrangement 112, they are coupled out from the opticalwaveguide 102 with diffraction. The case in which the light of the lightray 118 has a longer wavelength than the light of the light ray 116shall be assumed here. On account of the wavelength dependence ofdiffraction, the light ray 118 is diffracted at the input couplingarrangement 104 in the same order of diffraction with a largerdiffraction angle than the light ray 116, as is evident from FIG. 1.

In principle, it holds true that the exit pupil of a source image can beexpanded via diffractive input coupling of light into the opticalwaveguide arrangement 100, as is indicated by the region EEP (expandedexit pupil) in FIG. 1.

If the diffraction grating structures 108 and 112 are symmetrical withrespect to one another, the light rays 116, 118 are coupled out from theoptical waveguide 102 at an angle of emergence that is identical to theangle of incidence of the light rays 116 on the input couplingarrangement 104, as is shown in FIG. 1 for normal incidence of the lightrays 116, 118 on the input coupling arrangement 104. However, this alsoholds true for non-normal incidence on the input coupling arrangement104.

FIG. 1 depicts two regions 120 a and 120 b illustrated in a hatchedmanner, which are not usable for the input coupling of light. The region120 a cannot be used because light diffracted into this region cannotpropagate in the direction toward the output coupling arrangement 106.The region 120 b cannot be used because light diffracted into thisregion is incident on the surface 114 at an angle of incidence that isless than the critical angle 122 of total internal reflection, such thatlight diffracted into this region at least partly emerges from thesurface 114 and thus likewise cannot propagate to the output couplingarrangement without loss of intensity.

As is furthermore evident from FIG. 1, on account of the multiple outputcoupling, less light having the shorter wavelength (light ray 116) iscoupled out compared with light having the longer wavelength (light ray118). This leads to polychromatic source images being transmittedwithout color fidelity.

FIG. 2 shows an optical waveguide arrangement 124 with one diffractiveinput coupling arrangement 126 and two diffractive output couplingarrangements 128 and 130. Such an arrangement is described in thearticle by Tapani Levola cited in the introduction: “Diffractive opticsfor virtual reality displays”. This arrangement can be used for thebinocular transmission of a source image into a binocular target image.θ and φ denote the angles of an incident light ray 132 with respect toan axis y and an axis x, respectively.

In order that only the 0 and ±1st orders of diffraction are coupled invia the input coupling arrangement 126, the grating period d of thediffractive input coupling arrangement 126 satisfies the followingcondition:

d≤λ/(1+|α_(0,max)|),

wherein α₀=sin θ cos φ and λ is the wavelength of the light.

The 0 order of diffraction will fall below the critical angle of totalinternal reflection, and so the 0 order of diffraction cannot be used,while the +1st order of diffraction can propagate to the output couplingarrangement 128 and the −1st order of diffraction can propagate to theoutput coupling arrangement 130 and in each case be coupled out there.

α_(0,max) is thus a measure of the maximum angle of incidence of lighton the input coupling arrangement 126 for a predefined grating period dof the input coupling arrangement 126 if only the +1st or −1st order ofdiffraction is intended to be used. Higher orders of diffraction shouldbe avoided since they lead to ghost images.

As has been explained above with reference to FIG. 1, however, thediffraction angles are different for different wavelengths of the light.This also applies to the +1st order of diffraction and the −1st order ofdiffraction. This has the consequence that, as illustrated in FIG. 1,the step length of light rays of different wavelengths within theoptical waveguide 102 between in each case two successive total internalreflections is different.

If consideration is given to the wavelength range of the visiblespectrum extending from approximately 425 nm (blue) to approximately 675nm (red), the result is a not inconsiderable different step procedurebetween the total internal reflections of light rays in the red spectralrange and light rays in the blue spectral range.

FIG. 3 shows a diagram showing a coefficient R, which denotes themaximum ratio of the step length between total internal reflections of alight ray having a maximum wavelength (in the visible range) to the steplength between total internal reflections of a light ray having aminimum wavelength (in the visible range) within an optical waveguidefor different materials of optical waveguides as a function of the fieldof view (FOV) for different materials having different refractiveindices. The coefficient curves R for materials having refractiveindices n=1.49; n=1.53; n=1.59; n=1.71 and n=1.85 are illustrated. FIG.3 reveals that as the field of view increases, which is accompanied byincreasing angles of incidence on the input coupling arrangement, thecoefficient R increases, although the increase is smaller as therefractive index increases. R should not be greater than 5. For R≤5, itis possible to achieve only fields of view of approximately 15° for thesmallest refractive index of n=1.49 considered in FIG. 3, and fields ofview of approximately 32.5° for the largest refractive index of 1.85considered in FIG. 3.

For the refractive index n=1.71, which is that of the material MGC171,for example, it is possible to achieve a field of view of approximately25° at R=5 if light in the red spectral range and light in thegreen/blue spectral range are transmitted separately in separate,stacked optical waveguides.

The relationships of diffractive input coupling of light into an opticalwaveguide arrangement 136 and propagation of the light in the opticalwaveguide arrangement 136 via total internal reflection are generallyelucidated with reference to FIG. 4. For this purpose, FIG. 4 shows twoincident light rays 138 and 140, which are incident from a medium havinga refractive index no at angles of incidence α_(i,max) and α_(i,min) andare coupled with diffraction into the optical waveguide arrangement 136,which has a refractive index n₁.

Generally, for a light beam coming from a medium having the refractiveindex no and incident at the angle α_(i) into the optical waveguidearrangement 136 having the refractive index n₁ with diffractivetransmissive input coupling at the input coupling location A, theFresnel equation holds true:

${{n_{1}\mspace{14mu} \sin \; \alpha_{m}} = {{m\frac{\lambda}{d}} + {n_{0}\mspace{14mu} \sin \mspace{14mu} \alpha_{i}}}},$

wherein m is the order of diffraction, α_(m) is the diffraction angle inthe m-th order of diffraction, d is the grating period of the inputcoupling arrangement and λ, is the wavelength of the light.

For the 0 order of diffraction (m=0), the Fresnel equation reduces toSnell's law of refraction. In this case, α_(m=0) is the angle ofrefraction. For m=±1, ±2, . . . it is found that the diffraction angleα_(m) is proportional to the wavelength λ (the wavelength dependence ofthe refractive index n₁ is negligible by comparison).

In order that a light ray coupled into the optical waveguide arrangement136 can propagate by total internal reflection in the optical waveguidearrangement 136, a further condition is that the diffraction angle α_(m)is greater than the critical angle of total internal reflection α_(T).

The light ray 140 in FIG. 4 is incident at the maximum angle ofincidence α_(i,max) which is still just diffracted into the opticalwaveguide arrangement 136 at the diffraction angle am,max. As is evidentfrom FIG. 4, however, this light ray propagates along the surface of theoptical waveguide arrangement 136 and, consequently, can just no longerundergo total internal reflection. The light ray 138 is incident on theoptical waveguide arrangement 136 at the angle of incidence α_(i,min),and is diffracted into the optical waveguide arrangement 136 at thediffraction angle α_(m,min). However, the diffraction angle α_(m,min) isalready the critical angle of total internal reflection α_(T).Consequently, the light rays 138 and 140 represent the outermosttheoretical limiting rays that can be transmitted through the opticalwaveguide arrangement as a result of diffractive input coupling into theoptical waveguide arrangement 136. The angle between these two lightrays 140, 138 is the field angle spectrum, or the maximum field of viewFOV, which can accordingly be transmitted.

On account of the dependence of the diffraction angles on the wavelengthof the light and the condition that the input coupling of light into theoptical waveguide arrangement satisfies the condition of total internalreflection, it is found that the transmittable field angle spectrum andthus the field of view and also the transmittable wavelength spectrumare limited. For a material of the optical waveguide arrangement whichhas a comparatively high refractive index, as is the case for thematerial polycarbonate having a refractive index of 1.588, it is noteven possible to transmit the entire visible wavelength spectrum havinga spectral width of 255 nm (425 nm to 680 nm) with a field of view of0°.

This substantive matter is illustrated in FIG. 5, which illustrates thewavelength spectrum Δλ that is transmittable as a function of the fieldof view (FOV) for an optical waveguide arrangement composed ofpolycarbonate (n=1.588). The solid line shows the curve for thetheoretically transmittable wavelength spectrum Δλ, and the interruptedline shows the curve for the wavelength spectrum Δλ that istransmittable in practice, in the case of which an offset of 5° withrespect to the angles of incidence of the theoretically possiblelimiting rays (138, 140, see FIG. 4) was taken into account in relationto the theoretical curve. For the material polycarbonate, it is possibleto achieve a theoretical field of view of approximately 30° only withabsolutely monochromatic transmission, and a wavelength spectrum Δλhaving a spectral width of 250 nm can be transmitted even theoreticallyonly with a field of view of 0°. The limiting values in practice areeven still below both the values mentioned above, as is evident fromFIG. 5 (interrupted line).

The region depicted in a hatched manner in FIG. 5 indicates thatwavelengths in a spectral range having a width of 100 nm can betransmitted with a field of view of a maximum of 14°.

FIG. 6 shows a diagram illustrating the minimum refractive index n as afunction of the field of view to be transmitted if the entire visiblespectral range Δλ having a spectral width of 255 nm is intended to betransmitted.

The solid line shows the theoretical curve of the minimum refractiveindex n, and the interrupted line 144 shows the curve in practice. FIG.6 shows that refractive indices of significantly above 2 are used fortransmitting large fields of view with the full visible wavelengthspectrum. In order to transmit a field of view of 20° in the fullvisible wavelength spectral range, a refractive index of 2.16 (verticalline 146) is thus used in practice. However, this is not achievable withconventional materials for optical purposes. By way of example, a line148 in FIG. 6 shows the refractive index of PTU, an episulfide, and aline 150 shows the refractive index of polycarbonate, which generallytend to be suitable as optical materials. Furthermore, it should benoted that the use of materials having a refractive index ofsignificantly greater than 2 has the disadvantage that 2nd and higherorders of diffraction are also coupled into the optical waveguidearrangement, but this should be avoided, as already mentioned above, inorder to avoid ghost images.

FIG. 7 shows the diagram in FIG. 5 again, but now a region is hatchedwhich indicates that a wavelength spectrum Δλ having a spectralbandwidth of less than 50 nm can be transmitted with a field of view of20°.

In order to be able to transmit a source image polychromatically, thatis to say in the entire visible spectral range, consideration was givento using a dedicated transmission channel in each case for thetransmission of individual spectral ranges, in order to solve theproblem of different step lengths at different wavelengths. This isillustrated in FIG. 8.

FIG. 8 shows an optical system 160 for transmitting a source image,including an optical waveguide arrangement 162, a diffractive inputcoupling arrangement 164 and a diffractive output coupling arrangement166. The optical waveguide arrangement 162 has three optical waveguides168, 170, 172 in a stacked arrangement. A respective air space 169, 171is situated between the optical waveguides 168 and 170, 172, wherein theair spaces 169, 171 can however also be filled by a medium whoserefractive index is lower than the refractive indices of the opticalwaveguides 168, 170, 172.

The input coupling arrangement 164 has three input coupling elements174, 176, 178, each having diffraction grating structures.

The output coupling arrangement 166 has output coupling elements 180,182 and 184.

The input coupling elements 174, 176, 178 are each tuned to a specificwavelength range. The input coupling element 174 is tuned such thatlight (ray 186) in the red spectral range is coupled into the opticalwaveguide 168 in the first order of diffraction, while light in thegreen and blue spectral range is transmitted without diffraction. Theinput coupling element 176 is tuned to diffracting light (ray 188) inthe green spectral range, wherein the green light is correspondinglycoupled into the optical waveguide 170 in the first order ofdiffraction, while blue light is transmitted without diffraction, andthe input coupling element 171 is tuned to diffracting light (ray 190)in the blue spectral range into the optical waveguide 172.

The output coupling element 180 is tuned to diffractively coupling lightin the red spectral range out from the optical waveguide 168, while theoutput coupling elements 182 and 184 are tuned to transmitting the lightin the red spectral range without diffraction. The output couplingelement 182 is correspondingly tuned to diffractively coupling light inthe green spectral range out from the optical waveguide 170, and theoutput coupling element 184 is tuned to diffractively coupling light inthe blue spectral range out from the optical waveguide 172.

The input coupling elements 174, 176, 178 are furthermore tuned suchthat the light from the respective spectral range is coupled into theassociated optical waveguide 168, 170, 172 at approximately the samediffraction angle, such that the step size between individual totalinternal reflections within the optical waveguides 168, 170, 172 isidentical.

The order of the arrangement of the gratings 174, 176 and 178 for thediffraction of the different spectral ranges need not correspond to theorder in FIG. 8 and can be adapted to as desired, and be reversed, forexample.

Via the color multiplexing realized with the optical system 160 in FIG.8, although the entire visible wavelength spectrum of a source image canbe transmitted, the field of view of the optical system 160 is stillrestricted to an angular range of below 20°. This is evident from FIGS.5 and 7, which reveal that only a field of view of 20° can be achievedper transmission channel with a transmitted spectral range having aspectral width of 50 nm, and even only a field of view of 14° can beachieved with a transmitted spectral range having a spectral width of100 nm per channel.

A further narrowing of the individual spectral ranges is not desired inconnection with RGB displays as image generators and moreover, as shownin FIGS. 5 and 7, does not lead to sufficiently large field angles whereFOV>20°.

An exemplary embodiment of an optical system with which fields of viewof more than 20° can be achieved is described with reference to FIG. 9.

FIG. 9 shows an optical system 10 for transmitting a source image 12.The optical system 10 has an optical waveguide arrangement 15, in whichlight can propagate by total internal reflection. Furthermore, theoptical system 10 has a diffractive optical input coupling arrangement16 for coupling the light 14 emanating from the source image 12 into theoptical waveguide arrangement 15. The optical system 10 additionally hasa diffractive optical output coupling arrangement 17 for coupling thelight that has propagated in the optical waveguide arrangement 15 outfrom the optical waveguide arrangement 15. The input couplingarrangement 16 has a first diffractive input coupling element 18 and asecond diffractive input coupling element 19. The first diffractiveinput coupling element 18 is arranged at a first optical waveguide 20 ofthe optical waveguide arrangement 15, and the second diffractive inputcoupling element 19 is arranged at a second optical waveguide 21 of theoptical waveguide arrangement 15. In this case, the two opticalwaveguides 20 and 21 are arranged in a stacked manner.

In contrast to the optical systems described previously, the sourceimage 12 is spatially divided into at least two subfields 12 a and 12 b,wherein the first subfield 12 a is coupled into the optical waveguide 20via the first input coupling element 18, while the second subfield 12 bis coupled into the second optical waveguide 21 via the second inputcoupling element 19, separately from the first subfield 12 a.

In line with the division of the source image 12 into at least twosubfields 12 a, 12 b, into exactly two subfields 12 a and 12 b in thepresent exemplary embodiment, the field angle spectrum emanating fromthe source image 12 is also correspondingly divided into a plurality of,here two, field angle ranges 22 a and 22 b. Consequently, light 14 withfield angles from a first field angle range 22 a emanates from the firstsubimage 12 a, and light 14 with field angles from a second field anglerange 22 b emanates from the second subfield 12 b. By way of example, inthe case where the source image 12 is divided into two subfields, thefield angle spectrum emanating from the source image 12 can be dividedinto a first field angle range having field angles in a range of −α to0° and into a second field angle range having field angles in a range of0° to +α, wherein a field angle of 0° corresponds to a direction ofincidence of a light ray parallel to the optical axis OA of an imagingoptical unit 25 a, 25 b.

By way of example, the division of the source image is effected in sucha way that the first subfield 12 a contains field angles in a fieldangle range of 0° to +20°, while the second subfield 12 b contains fieldangles in a field angle range of −20° to 0°.

The first subfield 12 a is coupled into the optical waveguide 20 via thefirst input coupling arrangement 18, in which optical waveguide thefirst subfield passes by total internal reflection to a first outputcoupling element 23, via which the first subfield 12 a is coupled outfrom the optical waveguide 20.

The second subfield 12 b is coupled into the second optical waveguide 21via the input coupling element 19, in which optical waveguide the secondsubfield passes by total internal reflection to a second output couplingelement 24, via which the second subfield 12 b is coupled out from theoptical waveguide 21. After both subfields 12 a and 12 b have beencoupled out from the optical waveguide arrangement 15, the transmittedsubfields 12 a and 12 b are superimposed on one another and combined toform the full transmitted source image, wherein the field of view is thesum of the field angle ranges of the subimages 12 a, 12 b.

By splitting the source image 12 into a plurality of subfields, it isthus possible to realize the optical system 10 with a larger field ofview, with a field of view of 2×20°=40° in the present example. In otherwords, by dividing the source image 12 into a plurality of subfields 12a, 12 b, . . . , it is possible to transmit the full field anglespectrum of the source image 12.

While the source image 12 is divided into exactly two subfields in theexemplary embodiment shown, it is generally possible to divide thesource image 12 into N subfields, wherein N is an integer ≥2, whereinthe input coupling arrangement 16 then has N input coupling elements.

The concept of dividing the source image 12 into a plurality ofsubfields can then moreover advantageously be combined with the colormultiplexing described with reference to FIG. 8. This can be realized bydividing the first subfield 12 a in accordance with FIG. 8 with regardto its wavelength spectrum into two wavelength ranges (red andblue/green) or into three wavelength ranges (red, green, blue) asillustrated in FIG. 8, wherein per subimage 12 a, 12 b each wavelengthrange is assigned a dedicated input coupling element such as the inputcoupling elements 174, 176, 178 in FIG. 8, and correspondingly threeoptical waveguides, such as the optical waveguides 168, 170 and 172 inFIG. 8.

Overall, in the case of a division of the source image 12 with regard tothe field angle spectrum into two subfields 12 a, 12 b and in the caseof an additional division of the wavelength spectrum emanating from thesource image 12 into three wavelength ranges, this then results overallin six transmission channels including six optical waveguides and sixinput coupling elements. Likewise, a corresponding number of six outputcoupling elements are then provided for the six transmission channels.

Referring to FIG. 9 again, the input coupling elements 18 and 19 areoptimized with regard to their diffraction efficiencies in the +1st or−1st order of diffraction. The first input coupling element 18 for theinput coupling of the first subfield 12 a is optimized to the fieldangle range 22 a with regard to its diffraction efficiency in the firstorder of diffraction. The second input coupling element 19 for the inputcoupling of the second subfield 12 b is optimized to the field anglerange 22 b with regard to its diffraction efficiency in the first orderof diffraction, wherein here it is desirable to take account of theopposite orientation of the angles of incidence from the field anglerange 22 b with respect to the angles of incidence from the first fieldangle range 22 a relative to the optical axis OA. Moreover, the inputcoupling elements 18 and 19 are tuned such that the diffraction anglesof light rays from the two subfields 12 a, 12 b, which apart from thesign have the same angle of incidence on the input coupling elements 18and 19, are coupled into the optical waveguide 20 and 21 with the samediffraction angle, such that the step length of these light rays betweenthe total internal reflections are identical, as is shown in FIG. 9.

The output coupling element 23 is symmetrical with respect to the inputcoupling element 18, such that light rays incident from the subfield 12a on the input coupling element 18 at a specific field angle are coupledout from the output coupling element 23 at the same field angle. Thesame applies to the adaptation of the output coupling element 24 to theinput coupling element 19 for the second subfield 12 b. The symmetrymentioned above is achieved in particular by virtue of the fact that thegrating period of the output coupling element 23 is equal to the gratingperiod of the input coupling element 18. The output coupling element 24likewise has a grating period that is equal to the grating period of theinput coupling element 19.

Furthermore, the diffraction efficiency of the output coupling element24 with regard to the light coming from the first subfield 12 a isminimized such that the light coming from the subfield 12 a can passthrough the output coupling element 24 without being diffracted orsubstantially without being diffracted.

The spectral ranges and used grating periods in this case depend on therefractive index of the optical waveguide 21. Examples for polycarbonateare indicated below. If the spectral range chosen is the blue spectralrange having wavelengths of between approximately 430 nm and 470 nm,then this results in a grating period of approximately 300 nm for thefirst grating 18 and a grating period of approximately 390 nm for thesecond grating 19. Grating periods of 360 nm and 470 nm, respectively,result for the green spectral range having wavelengths of between 520 nmand 570 nm. For the red spectral range between 600 nm and 660 nm, bycontrast, the grating periods to be chosen lie in the region of 420 nmand 550 nm, respectively, for the first and second gratings 18 and 19,respectively. The grating periods of the output coupling gratings 23, 24should be designed respectively analogously to the input couplinggratings 18, 19.

In the exemplary embodiment shown in FIG. 9, the input coupling elements18, 19 and the output coupling elements 23 and 24 are configured in eachcase as transmissive input coupling and output coupling elements,respectively, and correspondingly have transmissive optical diffractiongrating structures.

However, it is likewise possible to provide, instead of transmissiveinput coupling elements and transmissive output coupling elements,reflective input coupling elements and/or reflective output couplingelements, which are then correspondingly arranged at the oppositesurfaces of the optical waveguides 20 and 21, respectively, in contrastto the arrangement shown in FIG. 9. Reflective blazed gratings, inparticular, prove to be advantageous since they have a virtuallyconstant diffraction efficiency in the first order of diffraction overthe field angle range of 0° to 20° and of −20° to 0°. The two fieldangle ranges can be equal in magnitude, but they can also be ofdifferent magnitudes. The two field ranges can adjoin or partiallyoverlap one another. The homogeneity of the overlapping range can be setvia the represented signal on the display.

The diffraction grating structures mentioned above, both in the case oftransmissive diffraction grating structures and in the case ofreflective diffraction grating structures, can be configured as blazedgratings or have trapezoidal or rectangular webs that are inclinedrelative to the grating base.

The division of the source image 12 into a plurality of subfields 12 a,12 b, . . . can be realized by providing a corresponding number ofidentical source images 12, wherein stops, for example, are used toachieve the effect that only light from one subfield emanates from eachof the source images.

A different possibility of dividing the source image 12 into subfields12 a, 12 b, . . . is shown in FIG. 10. In accordance with FIG. 10, thesource image 12 is divided into the subfields 12 a, 12 b, . . . viaoptical deflecting elements 30 a, 30 b, . . . . In this case, the sourceimage has to be provided only once.

For the rest, the description concerning FIG. 9 also applies to FIG. 10.

FIG. 11 shows a further exemplary embodiment of an optical system 10′,in which elements that are comparable or identical to elements of theexemplary embodiments in FIGS. 9 and 10 are provided with the samereference sign, supplemented by a “′”.

The optical system 10′ has an optical waveguide arrangement 15′ havingonly one optical waveguide 20′, which serves for transmitting both thesubfield 12 a′ and the subfield 12 b′. The subfield 12 a′ is coupledinto the common optical waveguide 20′ via a diffractive input couplingelement 18′ and the subfield 12 b′ is coupled into the common opticalwaveguide 20′ via a diffractive input coupling element 19′.

The output coupling arrangement 17′ has a plurality of diffractionoutput coupling elements 24′ and a plurality of diffractive outputcoupling elements 23′, which are arranged alternately along the opticalwaveguide 20′, as is shown by way of example in FIG. 11. The individualoutput coupling elements 24′ and 23′ should each have an extent ofapproximately 1 mm. The extent is limited by the pupil diameter of theobserver for an optical system worn on the head. The pupil approximatelyhas a diameter of 3-4 mm. In order to make both subimages visiblesimultaneously, the extent is chosen to be smaller than the pupildiameter (pupil division). On the other hand, the extent of the outputcoupling elements limits the optical resolution of the transmittedimage, such that the largest possible extent is desirable. It has beenfound that an extent of 0.6 mm-1.5 mm mediates an ideal compromise withregard to resolution and homogeneity of the visual impression.

In FIG. 11, the input coupling elements and output coupling elementseach have the same grating period. In a further embodiment, both outputcoupling elements 23′ and 24′ can be designed as a sinusoidal grating.This ensures that at every point the output coupling element couples outlight from both subfields 12 a′ and 12 b′ with an average efficiency.The above-discussed disadvantages of pupil division and homogeneity ofthe visual impression are thereby avoided. In this case, the exactgroove shape of the sinusoidal grating can vary symmetrically over theoutput coupling region in order to ensure homogenous output couplingover the entire output coupling region. Here the efficiency of theoutput coupling of the light rays of the subimage lying further away inthe light path is increased in each case; by way of example, the outputcoupling element is designed in the left region in a manner similar tothe shape of a blazed grating for the radiation of the right field half12 b′ (comparable to 24′), whereas it is designed in the right region ina manner similar to the shape of a blazed grating for the radiation ofthe left field half 12 a′ (comparable to 23′), like a symmetricalsinusoidal grating in the center and a respective continuous transitiontherebetween.

Overall, this gives rise to a symmetrical arrangement of the opticalsystem 10′ and, in combination with the color multiplexing in accordancewith FIG. 8, the number of optical waveguides of the optical waveguidearrangement 15′ is halved, that is to say that overall only threeoptical waveguides are used in the case where the wavelength spectrum isdivided into three separately transmitted wavelength ranges.

The optical waveguide arrangements 15 and 15′ described above areconfigured in each case as planar optical waveguide arrangements. Thissimplifies the calculation and optimization of the input couplingelements 18 and 18′, respectively, and 19 and 19′, respectively, and ofthe output coupling elements 23 and 23′, respectively, and 24 and 24′,respectively, with regard to their diffraction properties for the usedescribed above. For an integration of the optical systems 10 and 10′,respectively, into glasses worn by the user, which usually have curvedlenses, it is desirable, however, that the optical waveguidearrangements 15 and 15′, respectively, can be curved.

Exemplary embodiments of optical systems 40 including a curved opticalwaveguide arrangement 42 having a curved optical waveguide 44 are shownin FIGS. 12A to 12E. Light 47 coming from a source image (notillustrated) is coupled into the optical waveguide 44 via an inputcoupling arrangement 46 and is coupled out again from the opticalwaveguide via an output coupling arrangement 48. A correctionarrangement 50 is situated between the input coupling arrangement 46 andthe output coupling arrangement 48, the correction arrangement beingdesigned to correct geometric and/or chromatic aberrations of thewavefront that are caused by the total internal reflections along thecurved course of the optical waveguide arrangement 42.

The correction arrangement 50 here is a diffractive correctionarrangement having a diffraction grating structure. In the case of thecorrection arrangement 50, the correction effect on the wavefrontaberrations is of primary importance in the configuration. Thediffraction grating structure of the correction arrangement can becalculated in accordance with the imaging aberrations to be corrected.FIGS. 12A to 12E show various examples of correction arrangements 50.

In a first example in accordance with FIG. 12A of a correctionarrangement 50 designed only in sections, the correction arrangement 50compensates for the cumulative wavefront aberrations that arise at thealso plurality of reflections in the curved optical waveguide 44. Afurther correction arrangement 50 designed only in sections is shown inFIG. 12C. Here it is possible, for example, to design the correctiongrating 50 a for chromatic correction and the correction grating 50 forgeometric correction of the wavefront.

In a further example in accordance with FIG. 12B, the correctionarrangement 50 is embodied over the entire area of the curved opticalwaveguide 44. This enables a piecewise compensation of the wavefrontaberrations generated in the curved optical waveguide 44, for example ofthe wavefront aberration generated by the respectively directlypreceding or directly succeeding reflection in the curved opticalwaveguide. In particular, the correction arrangement 50 thus correctsaberrations that arise on account of the curved optical waveguide fromone output coupling location to the next (in this respect, also see theexamples in FIGS. 12D and E, in which a plurality of light rays coupledout are shown). Thus, within a region in which light is coupled out viaa plurality of output coupling locations, a complete correction of theimaging beam path is ensured and a corrected image of the source imageis offered to the observer at each output coupling location. FIGS. 12Dand 12E show examples of a correction arrangement 50 extending at leastin sections also over the output coupling region 48.

The correction arrangement 50 can be designed either for the innersurface (in relation to the radius of curvature) or for the outersurface or for both surfaces of the curved optical waveguide. The effectof the curved optical waveguide on wavefront aberrations and possiblyinduced chromatic aberrations can be corrected in the same grating or ina second grating of the correction arrangement.

On account of the correction element 50, the input coupling arrangement46 and the output coupling arrangement 48 can advantageously becalculated as for a planar optical waveguide arrangement such as isshown in the previous exemplary embodiments.

The above-described aspect of the present disclosure can be provided inthe exemplary embodiments in accordance with FIGS. 9 to 11, such thatthe optical waveguide arrangements 15 and 15′, respectively, can also becurved.

However, the present aspect can also be used independently of theaspects described in connection with FIGS. 9 to 11, in particular alsoin the case where the input coupling arrangements 46 and 48 are notdiffractive, but rather only refractive or reflective input couplingarrangements without a diffraction effect.

Further aspects and exemplary embodiments of optical systems fortransmitting a source image are described below with reference to FIGS.13 to 30. Insofar as the spatial assignments “top”, “bottom”, “lateral”,“left”, “right” are used hereinafter, these terms have been chosenmerely for reasons of simpler understanding. These spatial terms relateto the case where an optical system for transmitting a source image isworn by a user on the head, for example in the manner of glasses.

A limitation of an achievable field of view or field angle of 20°, forexample, is mentioned in each case in the above description of thedisclosure. This limitation applies, as evident from the abovedescription, in the direction in which the light is coupled into theoptical waveguide arrangement and is guided along the optical waveguidearrangement. In this direction, the guidance of the light is subject tothe conditions of total internal reflection, as has been describedabove. This direction is also referred to as the critical direction. Itgoes without saying that the field of view or the image field angle ofthe transmitted source image is limited only in the critical direction.In the other direction, perpendicular thereto, the image field angle canbecome larger.

FIG. 13 shows achievable image fields in the portrait format “3:4” andin the landscape format “16:9” for different input coupling directionsor critical directions, indicated by arrows.

For a representation in the portrait format with an aspect ratio of 3:4,upon input coupling from the right or left for typical materials of theoptical waveguide arrangement the result is a maximum image field ofapproximately 20°×27° with a diagonal of 33°. In this case, the criticaldirection is the horizontal direction. For a representation in thelandscape format with an aspect ratio of 16:9, upon input coupling fromthe top or bottom the result is a maximum image field or field of viewof 36°×20° with a diagonal field of view of 41°. In this case, thecritical direction is the vertical direction. The limit applies here ineach case to optical waveguide arrangements including optical waveguidescomposed of polycarbonates having a refractive index of 1.588; fields ofview that are larger approximately by 10° are achievable withepisulfides such as PTU having refractive indices of up to 1.78. Largerfields of view or field angles are achievable by virtue of the principleaccording to the disclosure of combining a plurality of field angleranges or subfields of the source image. In this regard, by way ofexample, approximately 60°×34° with a diagonal of 80° can be achieved inthe landscape format with the vertical direction as the criticaldirection.

FIG. 14 shows a basic schematic diagram of an optical system 200 fortransmitting a source image in an arrangement for the right eye in thecase of lateral horizontal input coupling of a source image. Light of adisplay or source image 202 is directed via an optical unit 204 onto aninput coupling element 208 arranged in an optical waveguide 206, isguided via total internal reflection through the optical waveguide 206and is coupled out into the user's right eye 212 via an output couplingelement 210. The light emanating from the source image 202 right intothe user's eye 212 is illustrated by an interrupted line 214. Acoordinate system 216 in FIG. 14 indicates the direction of the x-axisand of the z-axis. The latter indicates the viewing direction of theeye. In accordance with the previous definition, the x-axis and thez-axis span a horizontal plane. FIG. 14 is thus a plan view of thesystem 200 from the top toward the bottom.

FIG. 15 shows the field angles transmittable in the optical waveguide206 in the spatial frequency domain or k-space. The indices “x” and “z”relate to the coordinate system 216 in FIG. 14. The followingconsideration applies to a wavelength λ. The upper partial figure inFIG. 15 shows a section through the Ewald spheres in thek_(z)-k_(x)-plane, and the lower partial figure in FIG. 15 shows asection through the Ewald spheres in the vertical k_(y)-k_(x)-plane.

The small circle 218 having the radius 2π/λ represents in each case asection through the Ewald sphere of the light outside the opticalwaveguide 206 having a refractive index of 1. The large circle 220having the radius n2π/λ represents in each case a section through theEwald sphere of the light within the optical waveguide 206 having arefractive index n. In order that the light is guided by total internalreflection in the optical waveguide 206, the x-component of the K-vectorin the optical waveguide 206 is greater than 2π/λ. This critical angleis represented by a perpendicular dashed line 222. The guided angularspectrum of the light 214 then extends as far as a propagation angleparallel to the interface 224 (FIG. 14) of the optical waveguide 206.For practical reasons, for the guided light 214 a somewhat smallerangular range is chosen, for example as the smallest angle an angle thatis greater than the critical angle of total internal reflection by 5°,and as the largest angle an angle that is less than the angle parallelto the interfaces by 15°, as has already been described above forexample with reference to FIGS. 5 to 7. These two critical angles arerepresented by dotted lines 226, 228.

This results in an angular range in k-space which is guided in theoptical waveguide 206 and which is restricted in the critical direction(horizontal) to an angular range that is illustrated with hatched linesand provided with the reference sign 230 in the upper partial figure inFIG. 15. In the non-critical direction (x-y-plane, lower partial figurein FIG. 15) there are no restrictions since the light is guided by totalinternal reflection in every direction in the optical waveguide 206.

The angular spectrum guided in the angular range 230 through the opticalwaveguide 206 can then be diffracted out of the optical waveguide 206through a suitable output coupling element 210, for example in the formof a linear grating, and be fed to the observer's eye 212. This isillustrated in the further figures to be described below.

Firstly, FIG. 16 shows a wave vector K_(i), incident on the outputcoupling element 210 in the form of an output coupling grating. For thediffraction at the grating, the Laue equation is satisfied:

K _(s) −K _(i) =G

The wave vector K_(i) diffracted at the grating is therefore permittedto differ from the incident wave vector K_(i) only by a grating vectorG. For the case where the output coupling element 210 is a monofrequencylinear grating lying parallel to the surface 224 of the opticalwaveguide 206, the possible grating vectors respectively for an order ofdiffraction of the output coupling grating lie on a perpendicular line232. The diffracted wave vector K_(s) (arrow 234), proceeding from theend of the incident wave vector K_(i), ends on an intersection point ofthe Ewald sphere having the radius n2π/λ and the perpendicular line 232of the possible grating vectors. As a result of the refraction at theinterface 224 of the optical waveguide 206, the transverse component ofthe wave vector does not change, such that the wave vector of the lightoutside the waveguide 206 proceeding from the end of the incident wavevector K_(i) ends on an intersection point of the Ewald sphere havingthe radius 2π/λ and the perpendicular line 232 of the possible gratingvectors, as represented by an arrow 236. With this design in accordancewith FIG. 16, it is possible, as explained below, to represent the fieldangle transmitted by an optical waveguide such as the optical waveguide206 with an output coupling element 210 in the form of a linear grating.

In this respect, FIG. 17 shows the k-space representation as in FIG. 15,wherein the line 232 of the possible grating vectors and the lines 226and 228 of the critical angles of the transmitted field angle rangeoutside the optical waveguide 206 are additionally illustrated. Thelower partial figure of FIG. 17 illustrates the transmitted field anglerange on the basis of the k-space representation in projection. Thetransmitted field angle range results from the overlap of the projectionof the Ewald sphere having the radius 2π/λ, which is centered around thegrating frequency line 232, and the angular range guided in the opticalwaveguide 206 from FIG. 15. The field angle range is thus arcuatelydelimited at each side (in the direction of the x-axis) and overall hasa bent shape, as is shown by the hatched region in the lower partialfigure of FIG. 15. The hatched region represents the transmittable fieldangle range.

FIG. 18 shows a rectangle 240 having an aspect ratio of 16:9 (portraitformat), the rectangle being depicted in the transmittable arcuate fieldangle range 238. With the use of polycarbonate and the above-indicatedangular distances relative to the respective critical angle, arectangular field angle range having an aspect ratio of 16:9 isapproximately 20°×35°, which corresponds to an image diagonal ofapproximately 40°.

The previous considerations related to a specific grating period orgrating frequency of the output coupling grating 210 in FIG. 14. Thetransmittable field angle range can be altered via the design of theoutput coupling grating. This is shown in FIG. 19 for three differentoutput coupling gratings having three different grating periods ofrespectively 60% of the waveguide λ, 78% of the wavelength λ and 107% ofthe wavelength λ (in vacuum), wherein instead of the one opticalwaveguide 206 in FIG. 14 three optical waveguides are present, which arearranged one behind another in the direction of the z-axis (as shown forexample in FIG. 22), wherein one of the output coupling gratingsmentioned above is assigned to each of the optical waveguides.

FIG. 19 shows such an embodiment of triple multiplexing in the anglespace, that is to say division of the source image into three subfields,upon input coupling of the subfields from the side (as shown in FIG. 14for the non-split source image 202 or as shown in FIG. 22). In thiscase, FIG. 19 shows the example of an angle space diagram for the righteye as in FIG. 14. The upper row of partial figures in FIG. 19 shows theangle space consideration in a cross section relative to the respectiveoptical waveguide, and the lower row illustrates an angle considerationin the viewing direction (z-axis), that is to say perpendicular to therespective optical waveguide or the optical waveguide arrangement. Thecircles 220 are in each case the projections of the Ewald spheres within(large circles having the radius n2π/λ) and the circles 218 outside(small circles having the radius 2π/λ) the respective optical waveguide.The dashed perpendicular lines 232, 232′ and 232″ in the upper row ofpartial figures in FIG. 19 represent the frequency line of therespective output coupling grating. The input and output couplingdirections respectively extend toward the right and left in FIG. 19. Theexample of output coupling via a respective output coupling grating intransmission is illustrated. The directions toward the top and bottom inthe partial figures of the lower row in FIG. 19 are also referred to asthe conic diffraction directions. The respectively transmitted differentfield angle ranges in the lower three partial figures result from therespective intersection lines of the critical angles in the respectiveoptical waveguide and the small circles shifted by the gratingfrequency. The maximally transmittable field angle ranges become arcuatevia the utilization of conic diffraction.

As is evident from the lower row of partial figures in FIG. 19, thetransmittable field angle ranges for the three different grating periodsof the respective output coupling gratings in k-space are laterallyoffset with respect to one another.

As already mentioned, the illustration in FIG. 19 corresponds to theillustration in FIG. 17 for three different, but slightly overlappingfield ranges of a source image which are intended to be transmitted byoptical waveguides arranged one behind another with three differentgrating periods of the respective output coupling grating ofrespectively 60% of the wavelength, 78% of the wavelength and 107% ofthe wavelength (in vacuum). For a wavelength of 550 nm, for example,grating periods of the respective output coupling grating ofapproximately 330 nm, 430 nm and 590 nm thus result.

FIG. 20 shows, in the left partial figure, the three different, butslightly overlapping field ranges 242, 244 and 246, such as areperceived by the observer when they are joined together to form anoverall image.

The middle partial figure in FIG. 20 depicts a rectangle 248 having anaspect ratio of 16:9, which represents an entire field angle range or animage field or field of view which can be transmitted upon inputcoupling of three subfields corresponding to the field angle ranges 242,244, 246 of the source image from the side in a horizontal direction (asshown for example in FIG. 14 or 22). The total field angle range or theimage which can be transmitted in the case of this arrangement has asize of 62°×35°, which corresponds to an image diagonal of approximately71°.

The right partial figure in FIG. 20 shows with a rectangle 250 a totalfield angle range which is transmittable upon input coupling of threesubfields corresponding to the field angle ranges 242′, 244′, 246′ fromthe top or bottom, that is to say vertically and with an aspect ratio of16:9. The illustration in the right partial figure in FIG. 20 is rotatedby 90° for this. The total field angle range which can be transmitted inthe case of this arrangement is 100°×56°, which corresponds to an imagediagonal of approximately 115°.

Following these relationships, an optical system and a method accordingto the disclosure for transmitting a source image are based, then, onsplitting the source image into at least two subfields, at least one ofwhich is at least partly arcuately bounded before coupling into theoptical waveguide arrangement. Specifically, as is evident from theexplanations above, particularly large fields of view can be obtained ifthe field angle ranges are arcuately bounded at least at one side. Thisis illustrated schematically again in FIG. 21, for three field angleranges 252, 254, 256 which, when strung together, yield a maximum fieldof view. Since the field angle ranges transmittable for each opticalwaveguide are arcuate, it is possible to achieve a larger total fieldangle range during the transmission if the individual transmittableranges are fully utilized. This can be achieved according to thedisclosure if the field angle ranges adjoining one another are designedto be arcuate at least at a boundary of the individual field angleranges that is to be brought to overlap. The right partial figure inFIG. 21 illustrates in this respect the three field angle ranges 252,254, 256 which together yield the total field angle range of 62°×35°implemented in the left partial figure of FIG. 21 or in the centralpartial figure of FIG. 20. Consequently, as shown in FIG. 21 in theright partial figure, for example, the source image can be split intothree subfields, specifically the first subfield 252, the secondsubfield 254 and the third subfield 256, wherein a field edge 258 of thesubfield 252 and a field edge 260 of the subfield 254 and also a furtherfield edge 262 of the subfield 254 and a field edge 264 of the subfield256 are configured in each case as concavely or convexly arcuate. As isevident from the right partial figure of FIG. 21, the radii of thearcuate field edges 258 to 264 are not identical, such that thesubfields 252, 254 and 256 should partly overlap as shown in the leftpartial figure of FIG. 21. In this case, the field edges 258 and 260 aredirectly adjacent to one another, and so are the arcuate field edges 262and 264.

It goes without saying that the principle of splitting the source imageinto at least two subfields, of which at least one and preferably bothare arcuate on their field edges facing one another, can also be appliedto the exemplary embodiments in accordance with FIGS. 9 and 10, andlikewise to the exemplary embodiment in FIG. 11, wherein for the latteranother variant will be described later.

FIG. 22 shows one exemplary embodiment of an optical system 300 fortransmitting a source image 302, which likewise makes use of theabove-described principle according to the disclosure.

The optical system 300 in FIG. 22 is shown in plan view from above.

The optical system 300 has three optical waveguides 306 a, 306 b and 306c, which are arranged one behind another in the direction of the z-axis(see coordinate system in FIG. 22). An input coupling element 308 a andan output coupling element 310 a are assigned to the optical waveguide306 a. An input coupling element 308 b and an output coupling element310 b are assigned to the optical waveguide 306 b. An input couplingelement 308 c and an output coupling element 310 c are assigned to theoptical waveguide 306 c.

Each of the optical waveguides 306 a to 306 c transmits a subfield ofthe source image 302, that is to say a respective field angle range ofthe field angle spectrum of the source image 302, wherein the fieldangle ranges are at least partly different from one another, as is shownfor example in FIG. 21.

FIG. 22 shows the eye 312 of an observer who, as a result of thetransmitted subfields of the source image 302 being coupled out from theoptical waveguides 306 a, 306 b, 306 c, can perceive the complete sourceimage 302 through combination of the three transmitted subfields. Anangular range between approximately 45° and 75° of the guided light isin each case used in the optical waveguides 306 a, 306 b, 306 c. Theinput coupling elements 308 a, 308 b, 308 c and the output couplingelements 310 a, 310 b, 310 c are embodied as diffraction gratings,wherein the grating periods of the input coupling elements 308 a, 308 b,308 c are mutually different, and the grating periods of the outputcoupling elements 310 a, 310 b, 310 c are likewise mutually different.By contrast, the grating periods of the mutually associated outputcoupling elements and input coupling elements are identical. In theexemplary embodiment shown, the input coupling elements 308 a, 308 b,308 c and the output coupling elements 310 a, 310 b, 310 c are embodiedas buried diffraction gratings in the optical waveguides 306 a, 306 b,306 c.

Furthermore, optical units 304 a, 304 b and 304 c are shown in FIG. 22.In this configuration, the source image 302 is provided three-fold,corresponding to three identical source images 302 a, 302 b and 302 c.However, the full field angle range from each source image 302 a, 302 b,302 c is not coupled into the respective optical waveguide 306 a, 306 b,306 c, rather only a subfield that is arcuately bounded on one side oron both sides from each of the mutually identical source images 302 a,302 b, 302 c is coupled into the respective optical waveguide 306 a, 306b, 306 c. In other words, the source image 302 is split into threesubfields having different field angle ranges. The optical system 300correspondingly has a device 314 for splitting the rectangular sourceimage 302 into three subfields. For this purpose, the device 314 has anoptical arrangement including three field stops 314 a, 314 b and 314 c,wherein the field stops 314 a, 314 b and 314 c are arcuately bounded ina manner corresponding to the arcuate field edges 258, 260, 262, 264.

The field stop 314 a is shown by way of example in FIG. 23B). The fieldstop 314 a has an arcuate edge 316 a, via which the subfield 252 havingthe arcuate field edge 258 is generated. The field stop 314 b has anarcuate edge correspondingly on both sides in a manner corresponding tothe field edges 260 and 262, and the field stop 314 c has an arcuateedge, which serves for generating the subfield 256 having the arcuatefield edge 264.

As an alternative to generating arcuately bounded field angle ranges viafield stops, the division of the source image 302 into the subfields canalso be realized electronically by the corresponding pixels of therespective source image 302 a, 302 b, 302 c (in each case a display)that are not intended to be transmitted correspondingly not being drivenor being driven such that they remain dark. In FIG. 23A), the region ofthe non-driven pixels is provided with the reference sign 317.

Referring to FIG. 22 again, it should be noted that the thickness of theoptical waveguides 306 a, 306 b and 306 c can be very small inprinciple, for example 200 μm. For practical reasons, a larger thicknessmay be advantageous, for example a thickness of 500 μm for each of theoptical waveguides 306 a, 306 b, 306 c. The optical waveguides 306 a,306 b, 306 c should be mounted at a distance with an air spacing of atleast a few micrometers, for example 5 μm. This can be achieved viasmall spacers 320, which can be realized as small spheres or otherstructures on the optical waveguides 306 a, 306 b, 306 c.

In this way, even in the case of triple field stitching (that is to saydivision of the source image into three subfields) in conjunction withtriple color multiplexing, as was described with reference to FIG. 8,with then a total of nine optical waveguides, for example, relativelysmall thicknesses of less than 2 mm can be achieved. If the same opticalwaveguides in each case are used for the color multiplexing and thecolors are separated in each case via the embedded gratings, thethickness decreases further to less than 2 mm.

If, as shown in FIG. 20, right partial figure, input coupling of thesubfields of the source image 302 into the optical waveguides 306 a, 306b, 306 c from the top or from the bottom is realized, the transmittabletotal field angle range becomes even larger, as is evident from acomparison of the right partial figure in FIG. 20 with the centralpartial figure in FIG. 20.

If, by contrast, instead of arcuately bounded subfields, rectangularlydelimited subfields are used, as shown in FIG. 24, then in the case oftriple field stitching, that is to say division of the source image intothree rectangular subfields, upon input coupling from the top in eachoptical waveguide 306 a, 306 b, 306 c, only a field angle of a maximumof 80°×15° can be transmitted, and stringing together the three fieldangle ranges yields a resulting total field range of only 80°×45° with adiagonal of 92° by comparison with a field range of 100°×56° in the caseof the use according to the disclosure of arcuately bounded subfields orfield angle range segments. In the case of field stitching, it istherefore advantageous to transmit and string together arcuatesubfields. It is thus possible to transmit source images with largefield angles and thus a large field of view using fewer opticalwaveguides, for example only two or three optical waveguides.

FIG. 25 shows a further aspect of the disclosure. FIG. 25 shows threeoptical waveguides 326 a, 326 b and 326 c arranged one behind another inthe viewing direction and serving for transmitting three field anglesegments of a source image that adjoin one another, as described withreference to FIG. 22. The input coupling optical unit is not illustratedin FIG. 25. The optical waveguides 326 a, 326 b, 326 c illuminate theeyebox of the eye 328 of an observer, wherein the eyebox is the areathat is swept over by the eye pupil rolling over the entire field angle.In the case of the optical system 325, three output coupling elements330 a, 330 b, 330 c are provided, each of which respectively is assignedto the respective optical waveguide 326 a, 326 b, 326 c. In accordancewith this aspect of the disclosure, the output coupling elements 330 a,330 b, 330 c, which can be configured as diffraction gratings, arearranged in a manner offset with respect to one another in such a waythat only the eyebox in each case is illuminated for each opticalwaveguide 326 a, 326 b, 326 c and the field angle segment or field anglerange transmitted by the latter. The laterally delimited embodiment inan offset arrangement of the output coupling elements 330 a, 330 b, 330c thus achieves the effect that less light is lost.

For esthetic reasons, the optical waveguide arrangement including theoptical waveguides 326 a, 326 b, 326 c is furthermore arranged at aninclination in front of the eye 328, specifically by an angle of between10° and 20°, for example. Moreover, it is possible (not shown) toincline the planar optical waveguide arrangement including the opticalwaveguides 326 a, 326 b, 326 c in the other direction perpendicularly tothe plane of the drawing by the so-called pantoscopic angle of between10° and 20°. Larger angles of inclination in both directions aretechnically possible, but not preferable for esthetic reasons.

With more than three optical waveguides or materials having a relativelyhigh refractive index such as, for example, PTU (polyurethane) orepisulfides having refractive indices of up to 1.76, almost the completehalf-space can be transmitted by way of arcuate angular ranges adjoiningone another. Consequently, fields of view with a diagonal of more than115° are also possible. According to the disclosure, here a field anglesegment that is arcuately delimited in each case at at least one sidecan be transmitted in at least two channels, that is to say in at leasttwo optical waveguides.

As described above, for transmitting large field angle ranges with threecolors, for each field angle range and each color it is possible to usean optical waveguide with corresponding gratings. In the case of threefield angle ranges or segments and three colors, an optical systemaccording to the disclosure can therefore be realized with nine opticalwaveguides. Since the optical waveguides can be made thin, for example200 μm thin, it is thus possible to realize a thin optical waveguidearrangement having a total thickness of 2 mm for optical systems worn onthe head. However, the many interfaces, for example 18 in the case of 9optical waveguides, lead to high reflection losses. Therefore, it isadvantageous to antireflectively coat the optical waveguides withantireflection layers for the light passing through. The desiredproperties of antireflection layers can be reduced if, for each fieldangle range, the gratings for the three color ranges are embedded intoone optical waveguide. The number of optical waveguides thus decreasesto three with only six interfaces. Here for color multiplexing the sameoptical waveguides are used in each case and the colors are separated ineach case via different gratings having different grating periods forthe different wavelengths. The number of interfaces decreases as aresult, but nine gratings are used in the example explained.

For the polychromatic transmission, it is also possible to use anoptical waveguide with a coupling grating for a first field angle rangefor a first color and a second field angle range for a second color. Inthis regard, in polycarbonate with a grating period of 430 nm—in eachcase in the critical coupling direction—at a wavelength of 445 nm afield angle range of 0° to +20° can be transmitted, and at a wavelengthof 650 nm a field angle range of −20° to 0° can be transmitted. It isthus possible to reduce the number of used coupling gratings for atransmission of large field angles with three colors and to increase thetransmission of the overall system. By way of example, it is possible totransmit a large spectral range using only four coupling gratings andfour optical waveguides instead of nine coupling gratings in threeoptical waveguides.

A further embodiment of an optical system according to the disclosure isdescribed with reference to the further figures.

FIG. 26 shows an optical system 400 for transmitting a source image,which is configured similarly to the optical system 10′ in FIG. 11.

The system 400 serves for transmitting a source image 402 divided intotwo subfields 402 a and 402 b into an observer's eye 412. A respectiveoptical unit 404 a and 404 b is assigned to the subfields 402 a and 402b. The system 400 includes a single optical waveguide 406, into whichthe two subfields 402 a and 402 b are coupled from opposite sides of theoptical waveguide 406, for example from top and bottom or left andright.

Respective input coupling elements 408 a and 408 b are assigned to theoptical waveguide 406 correspondingly at opposite sides.

The construction of the system 400 is embodied symmetrically. Forcoupling out the two field angle ranges of the subfields 402 a, 402 b,here only one output coupling element 410 is used, that is to say thatthe output coupling element 410, for example in the form of adiffraction grating, serves for coupling out both transmitted subfields402 a, 402 b, that is to say the associated field angle ranges of thesource image 402 that are at least partly different from one another.

In order to realize this, only one grating period is used for the outputcoupling in the case of the output coupling element 410, although theshape of the grating structures of the output coupling element 410 isvariable over the extent thereof. In order to achieve homogenous outputcoupling of the light, it is advantageous, for the light coupled in fromthe left in FIG. 26, to cause the output coupling efficiency of theoutput coupling element 410 to increase from left to right and, for thelight coupled in from the right in FIG. 26, to cause the output couplingefficiency of the output coupling element 410 to increase from right toleft. This can be achieved as illustrated in FIG. 27 by the diffractiongrating of the output coupling element 410 being designed as asymmetrical sinusoidal grating in a central region 410 m, wherein theoutput coupling efficiencies for both input coupling directions (leftand right) are identical in the region, while the output couplingelement 410 assumes increasingly asymmetrical shapes of a diffractiongrating on both sides of the central region 410 m, which shapesincreasingly assume the shape of a blazed grating. This is shownschematically in FIG. 27 for a region 410 l and region 410 r, both ofwhich are arranged outside the central region 410 m. FIG. 27 likewiseshows schematically the fact that the direction of inclination of theincreasingly blazed grating in the outer regions 410 l and 410 r is inopposite senses with respect to one another.

FIG. 28 shows the two field angle ranges transmitted by the opticalsystem 400 in a manner corresponding to the subfields 402 a and 402 bgiven a specific grating period of the output coupling element 410 of498 nm and with polycarbonate as material of the optical waveguide 406for a wavelength λ of 550 nm. The transmitted or transmittable fieldranges are shown in a hatched manner and designated by the referencesigns 414 and 416 in FIG. 28. The representation in k-space inaccordance with FIG. 28 corresponds to a representation that is similarto FIG. 19, lower partial figures.

FIG. 29, left partial figure, illustrates the two combined field angleranges 414 and 416 as a projection of k-space. A rectangular field 418having an aspect ratio of 16:9 is depicted in the central partial figureof FIG. 29. A field angle or field of view of 65°×36° with a diagonal of75° is thus achievable.

In accordance with the concept according to the disclosure, it is alsopossible to transmit a larger field angle range if the field angleranges or at least one of the field angle ranges are or is at leastpartly arcuately bounded. This is illustrated on the basis of an examplein the right partial figure in FIG. 29 for the right eye. Here, by wayof example, the field angle range of the subfield 402 b or the subfield402 b is bounded by an arcuate field edge 420 at its lower side. Here,accordingly, unlike in the previous exemplary embodiments, the arcuateboundary is not effected at the mutually facing field edges of theindividual field angle ranges, but rather at an outer edge of at leastone of the subfields.

The configuration in accordance with the right partial figure in FIG. 29is desirable, for example, in order to magnify the field angle in theright half-space for the right eye.

At the same time, it is possible to magnify the field angle range forthe augmentation of the lower hemisphere by an arcuate boundary of thetransmitted field range. It goes without saying that further variantsare conceivable, such as a further magnification of the field of view byan arcuate boundary of the upper hemisphere, that is to say of thesubfield 402 a.

For the configuration with input coupling of the subfields from top andbottom or left and right, it is advantageous to place the input couplingon one side of the optical waveguide, as is shown in FIG. 30 for amodification of the optical system 400, this modification being providedwith the reference sign 450. The optical system 450 in turn includesonly one optical waveguide 456 having a first section 456 a, a secondsection 456 b and a third section 456 c. The sections 456 a and 456 brun parallel to one another, while the section 456 c preferablyintegrally interconnects the two sections 456 a and 456 b and runsperpendicularly to these two sections. The input coupling elements 458 aand 458 b are now arranged on the same side at the free ends of thefirst section 456 a and of the second section 456 b, respectively, ofthe optical waveguide 456.

The subfield 402 a of the source image is coupled into the first section456 a of the optical waveguide 456 via the input coupling element 458 a,wherein the light thus coupled in is guided via total internalreflection and in the third section 456 c via a reflectively coatedsurface 460 or retroreflectors present there into the second section 456b, in which the light is coupled out by the common output couplingelement 462 as in the case of the system 400 together with the othersubfield 452 b.

In this way, it is possible to achieve a large field angle range or alarge field of view using only one diffractive output coupling element462 per wavelength range and to increase the transmission of the opticalsystem 450, wherein at the same time the input coupling elements 458 aand 458 b and the associated optical units 454 a and 454 b are arrangedonly on one side of the optical transmission system, here of the opticalwaveguide 456. In the case of color multiplexing using different outputcoupling elements for the three colors red, green and blue in only oneoptical waveguide, according to the disclosure it is thus possible torealize a transmission element in the form of an optical waveguidecomposed of polycarbonate with a large field angle range of up to65°×36° using only one optical waveguide and three output couplinggratings in the transmission direction. Transmittable total field angleranges of 90°×51° with a diagonal of 103° can be realized in episulfidesor PTU having a refractive index of n=1.76.

What is claimed is:
 1. An optical system configured to transmit a sourceimage from which light having a field angle spectrum emanates, theoptical system comprising: an optical waveguide arrangement in which thelight can propagate by total internal reflection; a diffractive opticalinput coupling arrangement configured to couple the light emanating fromthe source image into the optical waveguide arrangement; and adiffractive optical output coupling arrangement configured to couple thelight that has propagated in the optical waveguide arrangement out fromthe optical waveguide arrangement, wherein: the input couplingarrangement comprises: a first input coupling element configured tocouple light from a first subfield of the source image having fieldangles from a first field angle range of the field angle spectrum intothe optical waveguide arrangement; a second input coupling elementconfigured to couple light from a second subfield of the source imagehaving field angles from a second field angle range; the second subfieldis at least partly different from the first subfield; the second fieldangle range is at least partly different from the first field anglerange; the transmitted first subfield and the transmitted at least onesecond subfield are at least partly superimposed on each other aftercoupling out from the optical waveguide arrangement; and at least onesubfield selected from the group consisting of the first subfield andthe second subfield is at least partly arcuately bounded before couplinginto the optical waveguide arrangement.
 2. The optical system of claim1, wherein: the first subfield has a first field edge; the secondsubfield has a second field edge; the first field edge is directlyadjacent to the second field edge; and one of the following holds: thefirst field edge is concavely arcuate and the second field edge isconvexly arcuate; and the first field edge is convexly arcuate and thesecond field edge is concavely arcuate.
 3. The optical system of claim2, wherein the first and second subfields partially overlap in a regionof the first and second field edges.
 4. The optical system of claim 2,wherein: the input coupling arrangement comprises a third input couplingelement configured to couple in a third subfield of the source image;the third subfield is arranged between the first and second subfields;the third subfield has third field edges of which one is directlyadjacent to the first field edge and the other is directly adjacent tothe second field edge; and both third field edges are arcuate.
 5. Theoptical system of claim 1, wherein at least one subfield selected fromthe group consisting of the first subfield and the second subfield isarcuately bounded at an outer field edge.
 6. The optical system of claim1, wherein: the input coupling arrangement has a number of N inputcoupling elements configured to couple light from N different subfieldsof the source image having field angles from N at least partly differentfield angle ranges of the field angle spectrum into the opticalwaveguide arrangement; and N is an integer greater than
 2. 7. Theoptical system of claim 1, wherein: the input coupling arrangementcomprises two first and two second input coupling elements; one of thetwo first and one of the two second input coupling elements areconfigured to couple light from the first and second subfields,respectively, in a first wavelength range into the optical waveguidearrangement; the other of the two first and second input couplingelements are configured to couple light from the first and secondsubfields), respectively, in a second wavelength range into the opticalwaveguide arrangement; and the second wavelength range is different fromthe first wavelength range.
 8. The optical system of claim 7, whereinthe optical waveguide arrangement comprises: a first optical waveguideconfigured to the propagate light in the first wavelength range; and aseparate second optical waveguide configured to propagate light in thesecond wavelength range.
 9. The optical system of claim 1, wherein theoptical waveguide arrangement comprises: a first optical waveguide intowhich the first input coupling element couples the light from the firstsubfield; and a second optical waveguide into which the second inputcoupling element couples the light from the second subfield.
 10. Theoptical system of claim 9, wherein the first output coupling element isconfigured to couple light out from the first optical waveguide, and thesecond output coupling element is configured to couple light out fromthe second optical wave-guide.
 11. The optical system of claim 1,wherein: the optical waveguide arrangement comprises an opticalwaveguide into which the first input coupling element couples the lightfrom the first subfield and the input coupling element couples in thelight from the second subfield; and the first and second input couplingelements are arranged in opposite end regions of the optical waveguide.12. The optical system of claim 11, wherein the first and second outputcoupling elements are configured to couple light out from the oneoptical waveguide.
 13. The optical system of claim 12, wherein: theoutput coupling arrangement comprises a plurality of first outputcoupling elements and a plurality of second output coupling elements;and the first and second output coupling elements are arrangedalternately along the optical waveguide.
 14. The optical system of claim11, wherein the output coupling arrangement comprises only one outputcoupling element.
 15. The optical system of claim 1, wherein: theoptical waveguide arrangement comprises an optical waveguide into whichthe first input coupling element couples the light from the firstsubfield and the second input coupling element couples the light fromthe subfield; the optical waveguide comprises two mutually parallelfirst and second sections and at a first end a third sectionperpendicular to the first and second sections; and the first and secondinput coupling elements are arranged at free second ends of the opticalwaveguide.
 16. The optical system of claim 1, wherein the outputcoupling arrangement comprises: a first output coupling elementconfigured to couple light from the first subfield of the source imageout from the optical waveguide arrangement; and a second output couplingelement configured to couple light from the second subfield of thesource image out from the optical waveguide arrangement.
 17. The opticalsystem of claim 1, wherein at least one member selected from the groupconsisting of the first input coupling element and the second inputcoupling element comprises a transmissive optical diffraction gratingstructure.
 18. The optical system of claim 1, wherein at least onemember selected from the group consisting of the first input couplingelement and the second input coupling element comprises a reflectiveoptical diffraction grating structure.
 19. The optical system of claim1, further comprising a device configured to divide the source imageinto the first and second subfields.
 20. A method of transmitting asource image from which light having a field angle spectrum emanates,the method comprising: dividing the light emanating from the sourceimage into first and second subfields, the light of the first subfieldhaving field angles in a first field angle range of the field anglespectrum, the light of the second subfield having field angles in asecond field angle range of the field angle spectrum which is at leastpartly different from the first field angle range, at least one subfieldselected from the group consisting of the first subfield and the secondsubfield being arcuately bounded; diffractively coupling in light of thefirst subfield and, separately from the input coupling of the light ofthe first subfield, diffractively coupling in the second subfield intoan optical waveguide arrangement; propagating the light of the first andsecond subfields in the optical waveguide arrangement; and diffractivelycoupling out the first and second subfields from the optical waveguidearrangement so that the transmitted first subfield and the transmittedsecond subfield are at least partly superimposed on one each other aftercoupling out from the optical waveguide arrangement.